Anatomy and evolution of the Astoin diapiric complex, sub-Alpine fold-and-thrust belt (France)

Abstract

The structure of the southwestern branch of the Alpine orogen is affected by the extensive Late Triassic evaporites. These evaporites have been involved in polyphased salt tectonics since the early Liassic, coeval with the Tethyan rifting, and are the décollement level for thrusts in the external parts during Alpine orogeny. The role of salt tectonics in this branch of the Alpine arc is re-evaluated in order to determine the relative importance of early deformation related to salt motion with respect to deformation related to main Alpine compressional events. This paper focuses on one structure identified as diapiric since the 1930’s: the Astoin diapir (Goguel 1939). Analysis of geological maps together with new field work have allowed to better define diapirism in the Upper Triassic evaporites outcrops around Astoin. Study of the diapir and the surrounding depocenters reveals a major involvement of salt in the structuration of the area, since the Liassic. Several salt ridges are linked to a main diapiric structure, explaining why we call it the “diapiric complex” of Astoin. Salt tectonics was initiated during the Liassic rifting, and a few locations show evidence of reactive diapirism whereas in others evidence of passive diapirism as early as the Liassic is seen. Passive diapirism continued during the post-rift stage of Alpine margin history in the Late Jurassic and Cretaceous when an allochthonous salt sheet was emplaced. Diapirism also occurred during the Oligocene while the Alpine foreland basin was developing in this part of the European margin of the Alps. Serial interpretative cross-sections have been drawn in order to illustrate the lateral variations of diapirism and structural style. Sequential evolutions for each cross-section are proposed to reconstruct the diapiric complex evolution through time. The Astoin diapir shows a complex structural framework with an important along-strike variation of diapiric activity. Most of the geometries are inherited from salt tectonics that occurred during extension, and in some places these early structures are overprinted by Alpine compressional structures.

Structural evolution of the superimposed Provençal and Subalpine fold-thrust belts (SE France)

“…In the Barles area, folding was recorded immediately after the Provençal compression by growth stratal geometries in the Oligocene continental Red Molasse Formation, then in the Grey Molasse Formation south of La Maurière anticline (Maillart et al, 1987). This Oligocene deformation is consistent with growth stratal geometries in the Esparron syncline as described by Gidon (1997) and Célini et al (2021) more to the north (Fig. 3).…”

“…20a). This eroded part might correspond to the adjacent halokinetic Turriers basin located to the west and transported ahead of the Digne Nappe along the studied cross-section (Gidon, 1982;Célini et al, 2020Célini et al, , 2021. The restoration suggests that the Digne basin was initially located at ~8 km to the north from the Barre de Chine and Barles diapir (Fig.…”

“…According to field data, the cross-section suggests small reactivation (less than 500 m of shortening) of the pre-existing halokinetic folds of the Barles area (Maillart et al, 1987;Gidon, 1997;Célini et al, 2021). This Oligocene deformation might be interpreted as distal shortening in the Oligocene Alpine foreland.…”

“…The pre-shortening restoration also suggests that fold structures observed in the footwall (Barles diapir and Barre de Chine flap, Les Sauvans minibasin and Barles folds) and hangingwall (La Robine) of the Digne Nappe have been initiated by early halokinetic deformations through the Jurassic and maybe until the Early Cretaceous (Fig. 20a; Graham et al, 2012;Célini et al, 2020Célini et al, , 2021. (Jakni, 2000;Bestani, 2015;Schwartz et al, 2017a).…”

“…During shortening, the pre-existing halokinetic structures were generally reactivated, localizing preferentially the shortening in the cover. The initial geometry of these pre-existing halokinetic structures can be strongly modified during the contractional deformations as documented, for instance, in the Pyrenees (Saura et al, 2016;Labaume and Teixell, 2020;Burrel and Teixell, 2021), in Provence (Angelier and Aubouin, 1976;Espurt et al, 2019b), in the Alps (Dardeau et al, 1990;Granado et al, 2018;Célini et al, 2020Célini et al, , 2021, in the Zagros (Callot et al, 2007(Callot et al, , 2012, and in the eastern Carpathians (Tămaș et al, 2021). Shortening induces the lateral and vertical evacuation of the mobile evaporitic material.…”